Method for controlling the eddy-current loss and increasing the permeability of magnetic alloys

ABSTRACT

A method of controlling the eddy-current loss and increasing the permeability of sulfur-and-manganese-bearing, 2-81 Molybdenum Permalloy, pressed powder by maintaining the alloy at an elevated temperature in an air or other environment for a sufficient time to begin nucleation of a solid-solution precipitate which is evenly distributed within the grains of the alloy, but without permitting significant growth and combination of the precipitate into large particles or their migration to the grain boundaries, and annealing in a reducing atmosphere for a period of time sufficient to cause secondary recrystallization of the grains and dissolve the solid-solution precipitate.

United States Patent Pingel 1451 Apr. 4, 1972 [72] Inventor:

[73] Assignee:

Vernon J. Pingel, Arlington Heights, 111.

Western Electric Company, Incorporated, New York, NY.

[22] Filed: June 27, 1969 {21] Appl. No.: 842,780

Related US. Application Data [63] Continuation-impart of Ser. No. 733,333, May 31,

1968, abandoned.

[56] References Cited UNITED STATES PATENTS Harris...

Given et a1: .1: 148/104 1,818,070 8/1931 Lathrop 148/104 2,531,445 11/1950 Laycock 148/104 2,977,263 3/1961 I-larendza-I-Iarinxma 148/104 3,086,280 4/1963 Gibbs et a] ..148/121 X 3,132,952 5/1964 I-Iarendza-Harinxma ..l48/ 104 X FOREIGN PATENTS OR APPLICATIONS 203,045 6/1955 Australia ..148/104 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White Attorney-W. M. Kain, R. P. Miller and A. C. Schwarz, Jr.

[57] ABSTRACT A method of controlling the eddy'current loss and increasing the permeability of sulfur-and-manganese-bearing, 2-81 Molybdenum Permalloy, pressed powder by maintaining the alloy at an elevated temperature in an air or other environment for a sufficient time to begin nucleation of a solid-solution precipitate which is evenly distributed within the grains of the alloy, but without permitting significant growth and combination of the precipitate into large particles or their migration to the grain boundaries, and annealing in a reducing atmosphere for a period of time sufficient to cause secondary recrystallization of the grains and dissolve the solid-solution precipitate.

9 Claims, 6 Drawing Figures Patented April 4, 1972 5 Sheets-Sheet 1 INVENTOR V. J. FHNGEL ATTORNEY Patented April 4, 1972 ""3 Shoets-Shoet S PRE C /P/ 7A 7' ON TE MPE RA TURE m w w w m3 $5 is X 33 .NEQWSU 6E METHOD FOR CONTROLLING THE EDDY-CURRENT LOSS AND INCREASING THE PERMEABILITY OF MAGNETIC ALLOYS RELATED APPLICATION This application is a continuation-in-part of applicant's copending patent application, Ser. No. 733,333, filed May 31, 1968 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the treatment of magnetic material for altering the electrical characteristics and for increasing the strength of a pressed powder magnetic alloy, and more particularly, to a method of processing magnetic alloy pressed powders including a precipitation treatment and an annealing which increases the permeability while simultaneously controlling and maintaining the eddy-current loss of the magnetic alloy at low levels.

2. Description of the Prior Art In telephony, the capacitance between the two wires of a transmission line causes distortion of the voice signals carried thereon. To compensate for this capacitance and thus neutralize the distortion, inductors (called loading coils) are connected in series with the transmission line approximately every mile. Such an inductor comprises coils of wire wrapped around a toroidal core formed of a magnetically permeable material. Such a core is commonly made by hot rolling an ingot of molybdenum-bearing-brittle Permalloy (an alloy of nickel and iron) until it breaks into fragments. The fragments are crushed to a fine powder of dust (hence, these cores are often called powder cores or dust cores) that is mixed with a small amount of talc (magnesium silicate) and annealed at apv proximately l,400 F. (760 C.) in a reducing atmosphere, in an operation called calcining. The particles of calcined powder are then individually coated with a refractory insulator (comprising various amounts of one or more of the following: magnesium silicate, aluminum silicate, sodium silicate, magnesium hydroxide, water, a wetting agent, and a lubricant) and pressed to form the toroidal magnetic core that is then annealed in a reducing atmosphere at l,200 F. (649 C.) for over 40 minutes to relieve the work hardness that the Permalloy received during pressing. This production process is more completely shown in US. Pat. Nos. 1,669,649 granted to C. P. Beath et al. on May 15, 1928, 2,105,070 granted to A. F. Bandur on Jan. 1 l, 1938 and explained on pages 144-146 of Ferromagnetism by R. M. Bozorth, Copyright 1951 by D. Van Nostrand Company, Inc. and in an article entitled Permalloy Powder Cores by A. J. Harendza-Harinxma, appearing on page 10 of the Jan., 1964 issue of The Wester Electric Engineer.

The annealing operation is intended to raise the permeability of the core to the desired degree; however, if the resistivity of the core is too low, the core may still be unusuable due to high eddy-current losses, which would excessively attenuate the transmitted telephone signal. If the core is pressed to hard, the refractory insulator may break down and permit excessive eddy-current loss. If not pressed hard enough, the core may be mechanically weak and break apart when handled.

It is an object of the present invention to decrease the eddycurrent loss of a magnetic material without significantly decreasing its permeability or mechanical strength.

It is further an object of the present invention to increase the strength of an inductor core made of a pressed, insulated,

' magnetic powder.

It is another object of the present invention to increase the resistivity of a magnetic material.

An additional object of the present invention is to control the eddy-current loss of a magnetic material and increase the permeability without significantly decreasing its mechanical strength.

It is another object of the invention to precipitation treat and anneal particles of a magnetic powder to form precipitate nuclei and recrystallize the grains.

It is a further object of the invention to precipitation treat a magnetic core to form nuclei from a solid solution after which the core is annealed to dissolve the nuclei and cause secondary recrystallization of the grains to control the eddy-current loss and increase the permeability.

SUMMARY OF THE INVENTION In accordance with this invention, the eddy-current loss and permeability of a magnetic alloy and the strength of an insulated pressed powder of the alloy are controlled by heating the alloy to a temperature below its solid solution temperature for a period sufficient to begin nucleation and even distribution of a solid-solution precipitate throughout the grains of the alloy, but not sufficient to permit significant growth and combination of the precipitate into large particles and their migration to the grain boundaries, and annealing in a reducing atmosphere to cause secondary recrystallization of the grains and dissolve the solid-solution precipitate nuclei.

BRIEF DESCRIPTION OF THE DRAWINGS A complete understanding of the invention may be had by referring to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. I is a reproduction of a photomicrograph, at a magnification of 625 diameters, of an etched surface of a molybdenum-Permalloy powder core that has excessive eddy-current loss;

FIG. 2 is a reproduction of a photomicrograph, at a magnification of 625 diameters, of an etched surface of a molybdenum-Permalloy powder core of the same batch as the core shown in FIG. I and originally having excessive eddy-current loss, but having been subjected to the treatment of the present invention, and subsequently having an acceptable value of eddy-current loss;

FIG. 3 is a graph of permeability and eddy-current loss of a sample core as a percentage of an initial value, as a function of the duration of heat treatment at a fixed, predetermined temperature;

FIG. 4 is a graph of permeability and eddycurrent loss of a sample core as a percentage of an initial value, as a function of the number of individual heat treatments of short duration at a fixed, predetermined temperature;

FIG. 5 is a reproduction of a photomicrograph, at a magnification of 625 diameters, of an etchedl surface of a molybdenum-Permalloy having been subjected to a precipitation treatment and subsequent annealing, :now having a higher permeability; and

FIG. 6 is a graph showing the percentage change over initial value in permeability and eddy-current loss of sample cores as a function of varied precipitation temperatures for a fixed predetermined time followed by an annealing at a fixed, predetermined time and temperature.

DETAILED DESCRIPTION It is well-known that molybdenum is added to Permalloy to increase its electrical resistivity and thus reduce its eddy current loss. Sulfur is added to embrittle the Permalloy to cause it to fracture upon rolling, and too much manganese in Permalloy makes it ductile. Because of the effects caused by the addition of the above-mentioned chemical elements, Permalloy used for making telephone loading coil cores usually is specified as having to 83 percent nickel, 15 to 17 percent iron, 1.75 to 2.45 percent molybdenum, and customarily contains 0.01 to 0.04 percent manganese and 0.01 to 0.02 percent sulfur. None of these quantities is considered especially critical except that there must be enough sulfur to assure that the Permalloy will break up during the last pass of the hot rolling operation in spite of the presence of the manganese which is difficult to remove completely.

The 2-81 molybdenum-permalloy (approximately 2' percent molybdenum and 81 percent nickel with a remainder of substantially iron) of approximately the composition mentioned above has long been used to make loading coil cores by the aboveementioned manufacturing method. However, many batches of these cores were found to exhibit high eddy-current loss in spite of acceptable values of permeability and to be too weak mechanically to permit production handling. The poor eddy-current performance was usually attributed to failure of the refractory insulation (around each of the Permalloy powder particles) that is intended to prevent eddy-current from flowing between the individual particles. Therefore, in the past, the unacceptable cores were broken up and were again put through the insulating and pressing operation or were simply remelted into an ingot. This represented considerable manufacturing loss.

It has been found possible to reduce the eddy-current loss of otherwise unacceptable cores and to increase their tensile strength by an additional heat treating operation, hereinafter referred to as precipitation treatment, provided that the chemical composition of the Permalloy satisfies one additional condition. If the ratio (by weight) of manganese to sulfur in the 2-81 molybdenum-Permalloy is held between 0.5 and 4.0, the subsequently described precipitation treatment operation will increase the resistivity of the Permalloy (reduce the eddycurrent loss) and will appreciably increase the tensile strength of its insulated, pressed powder without appreciably reducing its permeability. Lower ratios of manganese to sulfur make the Permalloy less and less responsive to this precipitation treatment operation, with reduction of this ratio below 0.5 yielding marginally satisfactory results. Higher ratios of manganese to sulfur (for example, 6.0, 10.0, or more) make the permalloy more responsive to precipitation treatment, but it is difficult to make the Permalloy exhibit brittle fracture upon rolling, if it has a manganese-to-sulfur ratio of significantly more than 40. Therefore, higher ratios are possible and may even be desirable, but Permalloy with a manganese-to-sulfur ratio in excess of 4.0 is not ordinarily useful in the manufacture of powder cores.

In one embodiment of the present invention a composition of 2-8l molybdenum-Permalloy is maintained in an atmosphere of air at a temperature of 770 F. (410 C.) for a period of 6 hours. Such treatment of samples of 2-81 molybdenum-Permalloy has resulted in a 30 to 80 percent reduction in eddy-current loss and only a l to 7 percent reduction in permeability treatment is usually stopped before the permeability drops more than 7 percent. This time and temperature of treatment is considered most practical, from a commercial standpoint. If the treatment of sample cores of a batch shows that the above-mentioned temperature and time of treatment might not be optimum, treatment over a wide range of temperatures and times is possible to accomplish a comparable result. An optimum range of temperatures is from 740 (393 to 795 F. (424 C.) with a permissible practical range extending from 385 (196 to 865 F. (463 C.). The range of times at which cores are maintained at temperature varies from about 600 hours at 385 F. to no time at all at 865 F. (bring the Permalloy up to temperature and back down again quickly which usually takes a minimum of minutes in the furnace). The presence of an air atmosphere is not believed to be essential to the practice of the present invention but is believed to permit more economical treatment of the alloy. In addition, it has been noted that while higher temperatures treatment can be accomplished more quickly, it usually causes greater loss of permeability. Lower temperature treatment will reduce eddy-current loss just as much as higher temperature treatment and will cause less reduction of permeability; however, treatment at lower temperatures requires more time. Therefore, 6 hours at 770 F. was considered an optimum production treatment for 2-81 molybdenum-Permalloy.

While the exact reason for the above-mentioned reduction of eddy-current loss .is not fully understood, it appears from microscopic analysis that this precipitation treatment brings about precipitation within the individual grains of the Permalloy, which raises its resistivity, and also increases the bond strength between insulated particles of the powder without significantly reducing the permeability of a core. Excessive treatment at these temperatures permits the precipitate to combine into larger bodies or to migrate to the grain boundaries, causing the resistivity of the Pennalloy to decrease again, which results in an increased eddy-current loss.

Referring now to the drawings and more particularly to FIG. 1, which shows the microstructure of a loading coil core, the thick, dark lines are refractory insulation separating the individual particles of Permalloy powder. The thinner lines are grain boundaries within a particle, and the large, black spots are impurities. The particular core pictured in FIG. 1 exhibited an unacceptably high eddy-current loss, but photomicrographs of initially acceptable cores do not differ significantly from FIG. 1.

FIG. 2 is a photomicrograph of a core from the same batch as the core of FIG. 1, which also had an unacceptably high eddy-current loss. This core was held at 800 F. (427 C.) for 6 /2hours (not an optimum treatment) and exhibited a 44 percent reduction in eddy-current loss and only a 6.7 percent reduction in permeability as a result of the treatment. This core was then acceptable.

In FIG. 2, a fine sprinkling of tiny bits of precipitate is visible through the individual particles of powder, with a slight denuding of precipitate at the boundaries of the insulated particles due to migration of the precipitate to the grain boundaries which occurred because the temperature of the treatment was a bit too high and the time of treatment was somewhat too long foroptimum results (yet well within the spirit and scope of the present invention). Greater improvement would have resulted had the treatment been at a slightly lower temperature or had the time of treatment been slightly shorter.

It is believed that as soon as precipitation begins, the precipitate immediately starts combining into ever-larger particles and starts migrating to the grain boundaries. However, as long as precipitation occurs faster than the precipitate can combine and migrate to the grain boundaries, eddy-current loss continues to be reduced. It appears that when combination and migration outpace precipitation, eddy-current loss begins to increase as with the core of FIG. 2. A significant" amount of combination and migration of precipitate occurs when the permeability of the alloy has been reduced or the eddy-current loss has again increased to an extent that the alloy is no longer suitable for the use intended in the case of a powder core, the intended use is in a telephone loading coil.

It appears that the precipitation takes place slightly below a phase transformation temperature of the Permalloy. This can be noted from the fact that the thin, grain boundary lines so evident in FIG. I, seen in FIG. 2, to be disappearing. Such grain growth would ordinarily result in an increase of both permeability and eddy-current loss, but the precipitation phenomenon seems to overcome the reduced resistivity of the larger grains and to compensate for the increased permeability associated with larger grain size.

Some dots similar to the precipitate of FIG. 2 can be seen in FIG. 1. It is believed that some slight precipitation occurs incidentally as the core is cooled from the l,200 F. annealing treatment mentioned above. These dots were previously throught to be impurities and were ignored, their effect upon eddy-current loss and permeability appears not to have been recognized. It appears that above or at this recrystallization temperature, minor constituents of the alloy (the dots of precipitate of FIG. 2) can exist in solid solution within the alloy microstructure (called a solid-solution temperature).

Because the ratio of manganese to sulfur in the alloy seems to influence its responsiveness to this treatment, it is believed that the precipitate comprises sulfur, manganese and compounds thereof, with the possible inclusion of some molybdenum.

While the 2-81 molybdenum-Permalloy treated at the above temperatures is one embodiment of the invention, it is believed that the phenomenon experienced is not dependent upon the exact proportion of nickel present in the alloy, and that the maganese-to-sulfur ratio of from 0.5 to 4.0 (or more) is equally applicable to alloys containing from approximately 70 percent to approximately 90 percent nickel. Still other proportions of nickel (extending as low as 40 percent) would possibly necessitate slight variations in the ratio of manganese to sulfur. The optimum range of time and temperature of treatment of these other alloys would vary from the optimum mentioned above but would be within the permissible range mentioned above except that the decreasing proportions in nickel are believed to cause the upper limit of the permissible temperature range to rise above 865 F.

Molybdenum-amounting to from 1.75 to 2.45 percent ofthe alloy composition was mentioned above, but it has been found that higher proportions of molybdenum yield better results. It is believed that an optimum range of molybdenum content for powder cores would extend from 2.25 to 3 percent, with a per missible range extending from 0.5 percent to at least 5 percent.

Referring now to FIG. 3, after insulated Permalloy powder has been pressed and annealed at l,200 F, the cores are tested and found to have initial values of permeability and eddy-current loss. These initial values are referred to as the 100 percent values on the ordinates of the graph. The abscissa of FIG. 3 represents increasing rime, progressing from left to right. Therefore, the initial permeability and eddy-current loss of a batch of cores is represented by that point of the two curves where they meet the leftmost ordinate. Upon treatment of the cores at an elevated temperature within the useful range mentioned previously (for example, 740 F.) the permeability and eddy-current loss of the cores are both reduced with increasing time as illustrated by the curves in FIG. 3.

The rightmost end of the abscissa of the graph of FIG. 3 represents that time at which the cores exhibit approximately their minimum eddy-current loss. This time varies with the temperature of treatment and will be shorter at higher temperatures. As treatment is continued beyond this point, the precipitate that appears in the crystal structure of the core (FIG. 2) begins combining and migrating to the grain bounda' ries faster than additional precipitation occurs, resulting in an increase in the eddy-current exhibited by the core, until the eddy-current loss may again equal or even exceed the initial value. At some intervening time, the permeability of the core also begins to increase. However, by this time the core is unusable because of excessively high eddy-current loss. The ordinate values shown in FIG. 3 are representative values and are not applicable to all cores but can be used as a means of comparison with the curves shown in FIG. 4.

It has been found that repeated short treatments at temperatures within the practical range mentioned above but separated by intervals of cooling, produce comparable reductions in eddy-current loss but with only 25 to 35 percent as much loss of permeability. As an illustration of this, a batch of cores is inserted into a furnace that is held at 780 F., and the cores are kept there for no more than minutes and then cooled. Successive repetitions of this treatment reduce the eddy-current loss and permeability of the cores along curves similar to those of FIG. 4, wherein the abscissa represents increasing numbers of alternate heating and cooling cycles rather than increasing time of treatment. Eddy-current loss is reduced approximately the same amount by steady treatment and successive short treatments; but successive, short-duration treatments (preferably at a slightly higher temperature) separated by intervals of cooling, resulting in approximately one third as much degradation of permeability as the steady treatment represented by FIG. 3. Lower-temperature treatments, of course, require more repetition to achieve a comparable result.

It is well-known that when insulated Permalloy powder cores are pressed, greater core strength results from increased pressing pressure. It is also known that increasing pressure also causes breakdown of the refractory insulation which coats each particle of Permalloy. If the cores are too weak,

they will not sustain average production handling; and considerable loss will result from the breakage of weak cores. Added pressure overcomes this defect and strengthens the cores, so that they can be handled in production without undue fear of breakage; but increasing pressure causes increased eddy-current loss, which makes the cores unacceptable. It has been found that the heat treatment described above has the added benefit of increasing core strength between 50 and 350 percent, and at the same time reduces the eddy-current loss of the cores.

While the physical mechanism by which the increase in core strength is achieved is not fully understood, the cores are not strengthened as much at lower treatment temperatures within the optimum range as they are by treatment at higher temperatures. This suggests that besides the straining of the crystal lattice resulting from the precipitation phenomenon, the recrystallization shown in FIG. 2 may remove and put into solid solution some of the constituents that embrittle the grain and particle boundaries; and the treating temperature, although well below the e 1,200 F. annealing temperature, may even cause slight fusing of the refractory insulation that surrounds each particle of the Permalloy.

In another embodiment of the present invention, a composition of 2-81 molybdenum-Permalloy is precipitation treated over a range of temperatures for specified times afterwhich it is annealed. Such treatments of samples of 2-81 molybdenum- Permalloy have resulted in controlled eddy-current loss and increased permeability. More specifically, it has been found that selected low temperatures for precipitation treatment of Permalloy followed by an annealing in a reducing atmosphere maintains the eddy-current loss at the initial value while there is a marked increase in the permeability. In addition, it has 1. PRECIPITATION TREATMENT a. At Low Temperature The precipitation treatment of this embodiment of the invention is designed to maintain the eddy-current loss at a constant value and increase the permeability. This treatment comprises heating of the 281 molybdenum-Permalloy sample in an oxidizing, reducing or neutral atmosphere at a temperature of 590 F. (310 C.) for a period of 5 minutes. The range of times at which cores are maintained at temperature varies from about 5 hours at 240 F. (1 16 C.) to 15 minutes at 730 F. (388 C.) (bringing the Permalloy up to a temperature which usually takes a minimum of 15 minutes). The atmosphere is not critical to the practice of the present invention, however, an air atmosphere is normally used and permits more economical treatment of the alloy.

b. At High Temperature While it is desirous to produce cores with high permeability and low eddy-current loss, in certain instances the eddy-current loss can be disregarded or a higher value of eddy-current loss is permissible. In these cases, the precipitation treatment is carried out at temperatures above 730 F. (388 C.) in an atmosphere of air for 15 minutes. This 15-minute time interval is the normal time required to bring the Permalloy up to the furnace temperature. The range of time and temperature is dependent on the maximum permeability value desired, and the desired eddy-current loss characteristics. As an illustrated example, if a permeability of 218 with a core loss of below 10 is desired then the core will be precipitated treated at 820 F. (433 C.) for one minute at temperature.

0. Using Cyclic Technique The cyclic precipitation treatment that was discussed previously in the first embodiment is also applicable to this embodiment; however, such treatment should. be carried out at a lower temperature for shorter periods of time.

2. Annealing Following the precipitation treatment, the 2-81 molybdenum-Permalloy is annealed in a reducing atmosphere at a temperature of l,200 F. (640 C.) for a period of 30 minutes. The optimum range of temperature is from 1,150 (621 to 1,250 F. (677 C.), with a permissible, practical range ex tending from 1,l (593 to 1,300 F. (704 C.). The range of time varies from about 1 hour at 1,100 F. (593 C.) to no time at all at 1,300 F. (704 C.) (bringing the Permalloy up to temperature which usually takes minutes in the furnace). These annealing steps should be practiced in the presence of a reducing atmosphere. In addition, it has been noted that while precipitation treatment temperatures below 680 F. (360 C.) followed by annealing do not alter the initial eddy-current loss, they appreciably affect the permeability value. More specific, precipitation treatment below 680 F. (360 C.) fol lowed by annealing maintains a constant initial eddy-current loss value while raising the permeability over similar cores that have not had the precipitation treatment prior to annealing.

Furthermore, it has been found that precipitation treatment above 680 F. (360 C.) followed by annealing raises both the initial eddy-current loss value and the permeability. Therefore, in the manufacture of the cores requiring a higher permeability and a low eddy-current loss, precipitation treatment at 590 F. (310 C.) for 5 minutes is followed by annealing at l,200 F. (640 C.) for 30 minutes to obtain optimum values. With core requirements of high permeability and either a high value of eddy-current loss or no set value limitation, the precipitation treatment may be carried out at temperatures above 680 (360) for selected times ranging from a finite time interval to 1 minute followed by annealing. Excellent results may be obtained by annealing over a range of temperatures extending from 1,100 (593 to l,300 F. (704 C.); however, subjecting the cores to an annealing temperature of l,200 F. (640 C.) for 30 minutes leads to optimum results.

In certain instances with different batches of cores, or with cores having different percentages of elemental constituents, the above temperature ranges and times of treatment may not provide the desired results. In those instances, experimentation may be necessary by practicing both the precipitation treatment and annealing steps at different temperatures and for different time durations.

The exact reason for the above-mentioned changes in eddycurrent loss and increases in permeability are not fully apparent, but it appears from microscopic analysis that a solidsolution precipitate nuclei forms at temperatures below 780 F. (388 C. It is the formation of these nuclei that aid in holding the eddy-current loss near its initial value. In the subsequent annealing, secondary recrystallization occurs which results in larger grains. These larger grains raise the permeability. Also during the annealing, the formed nuclei of the precipitation treatment dissolve and during the cooling part of the annealing process, new nuclei of solid precipitate are formed. The composition of these new nuclei, it is believed, are related and controlled by the nuclei formed during the precipitation treatment.

At high temperatures, namely above 780 F. (388 C.), the solid-solution precipitate nuclei starts to form as the temperature increasesqhowever, they readily dissolve because 780 F. is above the solid-solution temperature. As before, the subsequent annealing causes secondary recrystallization. However, in this instance, the secondary recrystallization results in larger and more secondary grains which yields a higher increase in permeability.

The reason for these larger secondary grains and their increased number are due to the high precipitation treatment temperature. Upon cooling after the annealing, the solid-solution precipitate forms; however, this should be distinguished from those found in cores that have been low temperature precipitated, followed by an annealing. As mentioned above,

it is believed the nuclei formed during low temperature precipitation treatment control the composition and formation of those nuclei formed during annealing. However, the solid-solution nuclei are dissolved at high temperatures; therefore, the nuclei that form during cooling from the annealing temperature are independent of any previous nuclei. It is believed that this independence raises the eddy-current loss above its initial value.

The time and temperature of this embodiment of the invention are selected to form nuclei from the solid solution during the precipitation treatment and to cause secondary recrystallization of the grains and dissolving of the nuclei during annealing.

Referring now to the drawings and more particularly to FIG. 1, which shows the microstructure of a loading core, the thick dark lines are refractory insulation separating the individual particles of the Permalloy powder. The thinner lines are grain boundaries within a particle, and the large black spots are impurities.

FIG. 2 is a photomicrograph of a core from the same batch as the core of FIG. 1, this core was held at 800 F. (427 C.) for 6 V2 hours (not an optimum temperature).

FIG. 5 is a photomicrograph of a core that has been precipitation treated for 7 hours at 770 F. followed by an annealing in a reducing atmosphere of 1,200 F. for 30 minutes. The precipitation treatment times and temperature are not op timum; however, this photomicrograph was selected because it clearly illustrated the results of precipitation treatment followed by an annealing.

As mentioned earlier, FIGS. 1 and 2 are from the same batch; however, FIG. 5 is from a separate and distinct batch but photomicrographs of the cores from the batch of FIG. 5 taken at the same time in the process as those of FIGS. 1 and 2 would not differ significantly.

FIG. 6 shows the graphic change in the initial value of eddycurrent loss and permeability of cores subjected to precipitation treatment. After insulated powder has been pressed and annealed at 1,200" F. for 30 minutes in a reducing atmosphere, the cores are tested and the initial values of permeability and eddy-current loss are ascertained. An examination of the graph, indicates that treatments at temperatures below 240 F. do not result in significant changes in eddy-current loss and permeability. The abscissa of FIG. 6 represents precipitation treatment temperatures increasing from left to right. The time or precipitation treatment was 1 minute at any selected temperature. After the precipitation treatment the cores were annealed in a reducing atmosphere for 30 minutes at 1,200 F. The permeability and eddy-current loss values were tested and percentage change recorded.

In FIG. 6, it will be noted that the eddy-current loss value has 0 percent change at 680 F. and sharp percentage increase above 680 F. In addition, it will be noted that the permeability although cyclic throughout the range of temperatures is at all temperatures above the initial value and attains its highest percentage increase in the range of temperatures above 760 F.

At temperatures above 680 F., more specifically at 770 F. there is an increase in both the eddy-current loss and permeability value. It is believed that this is a result of the solid-solution precipitation nuclei starting to combine and migrate to the grain boundaries, and a steady increasing of temperature causes the solid-solution precipitate to dissolve. After the annealing, the grains go through a secondary recrystallization which is best illustrated by referring to FIGS. 1 and 5. It can be seen in FIG. 1 that the grain boundaries are small and well defined; however, in FIG. 5 there are large grains formed, namely, A. It is these large secondary recrystallization grains that raise the permeability. The eddy-current value increase is believed due to the lack of a finely dispersed solid-solution precipitate after the high temperature annealing treatment.

At temperatures below 680 F the solid-solution precipitate is formed. Such precipitate can be seen in FIG. 2 as dots evenly distributed throughout the microstructure. After run- the annealing, the grains go through a secondary recrystallization but not to the extent that they would have, had the core been precipitation treated at a higher temperature. However, the increase in grain size is sufficient to increase the permeability. The eddy-current loss value remains unchanged due to the fact that solid-solution precipitate was formed in the grain structure during the precipitation treatment and did not combine nor move to the grain boundaries. It is this presence of submicroscopic precipitation formed throughout the grains on various atom lattice sites at low temperatures that causes the increase in permeability during the annealing treatment due to secondary recrystallization.

In further discussions of this embodiment of the invention, reference will be made to manufacturing cores having high permeability and unchanged eddy-current loss value. Although very little mention will be made to producing cores having high permeabilities and high core loss, due to the fact that they have limited commercial use annealing the present time, it will be understood that the high precipitation treatment temperatures substituted for the lower precipitation treatment temperatures yield a higher eddy-current loss and a higher permeability.

While the above heat treatments, namely the precipitation treatment and annealing, have to be used together, a varied combination can be easily made by insertion at a different point in the process of magnetic core making as discussed earlier and which is reviewed now and lettered for later reference in the suggested alternate methods.

In general, Permalloy cores are made in accordance with the following process: (a) an ingot of molybdenum-bearingbrittle Permalloy is hot rolled until it breaks into fragments; (b) the fragments are crushed to a fine powder or dust that is mixed with a small amount of talc; (c) the fine powder and talc are annealed at approximately 1,400 F. in a reducing atmosphere called calcining; (d) the particles of calcined powder are then individually coated with a refractory insulator; (e) the insulated particles are pressed to form a toroidal magnetic core; and (f) the toroidal magnetic core is annealed in a reducing atmosphere at l,200 F. (649 C.) for about 30 minutes to relieve the work hardness that the Permalloy received during pressing.

Various Combinations of Precipitation an Annealing Steps Embodiment One Following the completion of all steps as outlined above, the core is subjected to precipitation treatment at a temperature of 590 F. for minutes which is subsequently followed by another annealing step in a reducing atmosphere at a temperature of l,200 F. (649 C.) for a period of 30 minutes. The resultant core exhibits no change in eddy-current loss values and an increase in permeability in comparison to cores treated by the prior art process. For example, with cores treated in accordance with this embodiment of the invention, the eddycurrent losses remained unchanged while the permeability increased by 10 percent.

Embodiment Two Following the first 5 steps of the method as outlined in the prior art, namely, steps, a, b, c, d, e, the pressed, insulated and calcined particles are pressed into form and precipitation treated to a temperature of 590 F. for 5 minutes followed by an annealing in a reducing atmosphere at a temperature of 1,200 F. for a period of 30 minutes to relieve the work hardness from pressing, recrystallize the grain, and dissolve the precipitation nuclei. This combination of steps, namely, the precipitation treatment prior to the step of annealing produces a core having unchanged eddy-current loss value and high permeability. The results were substantially the same as those obtained with the process described with respect to embodiment one.

Embodiment Three Again making reference to the steps outlined above, the precipitation treatment is carried out following the completion of steps a, b, c. More particularly, the calcined powder is precipitation treated at a temperature of 590 F. for a period of 5 minutes. The heat treated particles are subsequently covered with insulation, pressed into core form and annealed. Again, results were obtained which were substantially the same as those results obtained with the processes of embodiment one and two.

Each of the previous examples illustrates a method of producing cores with unchanged eddy-current loss values and high permeability. As mentioned earlier, these core values are most desirable as telephone loading coils; however, should the eddy-current value be of no consequence then the precipitation treatment can be carried out above 680 F. in which case each core will have an increased permeability and an increased eddy-current loss value.

What is claimed is:

l. A method of increasing the resistivity and strength of a molybdenum-Permalloy magnetic core, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid solution precipitate therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises:

heating the annealed core to a temperature within a range of 385 to 865 F. for less than 600 hours but more than 15 minutes to form and evenly distribute a solid-solution precipitate throughout the grains of the annealed core but not sufficient to permit significant combination of the solid-solution precipitate or migration of the solid-solu tion precipitate to the grain boundaries.

2. A method of treating a molybdenum-Permalloy material to increase its permeability while holding its eddy-current loss substantially constant, the material having a manganese-tosulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises:

heating the Permalloy material at a temperature sufficient to form solid-solution precipitate particles:

removing the material from the heat in time to preclude combination of the precipitate particles and the subsequent migration of the precipitate particles to the grain boundaries; and

annealing the material at a temperature for a time sufficient to recrystallize the grains. 3. A method of treating a molybdenum-Permalloy material to increase its permeability while holding its eddy-current loss substantially constant, the material having a manganese-tosulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises:

heating the molybdenum-Permalloy material at a temperature of 590 F. to form solid-solution precipitate particles;

removing the material after a period of at least 15 minutes to preclude combination of the precipitate particles and the subsequent migration of the precipitate particles to the grain boundaries; and

annealing the material at a temperature for a time sufficient to recrystallize the grains.

4. The method according to claim 3 wherein the material is annealed at 1,200 F. for a period of at least 30 minutes.

5. A method of treating a molybdenum-Permalloy material to increase its permeability and decrease its resistivity, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises:

heating the molybdenum-Permalloy material to a temperature of at least 730 F. to form solid-solution precipitate particles;

maintaining the temperature for a time period sufficient to dissolve the solid-solution precipitate particles;

removing the material from the heat upon dissolving the precipitate particles; and

annealing the material at a temperature for a sufficienttime to recrystallize the grains.

6. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been fonned by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises:

heating the particles at a temperature sufi'icient to form solid-solution precipitates; and

removing the particles from the heat in time to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.

7. A method of increasing the permeability of molybdenum- Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-tosulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises:

heating the particles at a temperature of 590 F. to form solid-solution precipitates; and

removing the particles from the heat after a period of at least minutes to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.

8. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises:

heating the pressed insulated particles to form solid-solution precipitates; and removing the particles from the heat in time to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.

9. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises:

heating the pressed insulated particles to a temperature of 590 F. to form solid-solution precipitates; and

removing the particles from the heat after a period of at least 5 minutes to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries. 

2. A method of treating a molybdenum-Permalloy material to increase its permeability while holding its eddy-current loss substantially constant, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises: heating the Permalloy material at a temperature sufficient to form solid-solution precipitate particles: removing the material from the heat in time to preclude combination of the precipitate particles and the subsequent migration of the precipitate particles to the grain boundaries; and annealing the material at a temperature for a time sufficient to recrystallize the grains.
 3. A method of treating a molybdenum-Permalloy material to increase its permeability while holding its eddy-current loss substantially constant, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises: heating the molybdenum-Permalloy material at a temperature of 590* F. to form solid-solution precipitate particles; removing the material after a period of at least 15 minutes to preclude combination of the precipitate particles and the subsequent migration of the precipitate particles to the grain boundaries; and annealing the material at a temperature for a time sufficient to recrystallize the grains.
 4. The method according to claim 3 wherein the material is annealed at 1,200* F. for a period of at least 30 minutes.
 5. A method of treating a molybdenum-Permalloy material to increase its permeability and decrease its resistivity, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitate particles therein, which comprises: heating the molybdenum-Permalloy material to a temPerature of at least 730* F. to form solid-solution precipitate particles; maintaining the temperature for a time period sufficient to dissolve the solid-solution precipitate particles; removing the material from the heat upon dissolving the precipitate particles; and annealing the material at a temperature for a sufficient time to recrystallize the grains.
 6. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises: heating the particles at a temperature sufficient to form solid-solution precipitates; and removing the particles from the heat in time to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.
 7. A method of increasing the permeability of molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises: heating the particles at a temperature of 590* F. to form solid-solution precipitates; and removing the particles from the heat after a period of at least 5 minutes to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.
 8. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises: heating the pressed insulated particles to form solid-solution precipitates; and removing the particles from the heat in time to preclude combination of the precipitates and the subsequent migration of the precipitates to the grain boundaries.
 9. A method of increasing the permeability of a molybdenum-Permalloy magnetic core while holding the eddy-current loss substantially constant, the core having been formed by hot rolling an ingot of molybdenum-Permalloy material until it breaks into fragments, the material having a manganese-to-sulfur ratio of about 0.5 to 4.0 to form solid-solution precipitates therein, crushing the fragments into powder, calcining the powder to form particles, coating the particles with an insulating material, pressing the insulated particles into a core, and annealing the core, which improvement comprises: heating the pressed insulated particles to a temperature of 590* F. to form solid-solution precipitates; and removing the particles from the heat after a period of at least 5 minutes to preclude combination of the precipitates and the subsequent migration of the precipiTates to the grain boundaries. 